Magnesium alloy and method for making the same

ABSTRACT

Magnesium-based alloys having good mechanical properties, such as mechanical strength, ductility, and castability, and methods of making the magnesium alloys are disclosed. The magnesium alloys comprise 8.7 to 11.8 wt % aluminum, 0.63 to 1.93 wt % zinc, 0.1 to 0.5 wt % manganese, 0.5 to 1.5 wt % rare earth elements, and a remainder of said magnesium alloy being composed of magnesium and unavoidable impurities.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is a divisional application of U.S. patent application Ser. No. 12/533,011, filed on Jul. 31, 2009, the entire content of which is incorporate by reference.

BACKGROUND

1. Technical Field

The present disclosure relates to magnesium-based alloys having good mechanical properties, such as mechanical strength, ductility and castability.

2. Description of the Related Art

Magnesium-based alloys have been widely used as cast parts in the aerospace and automotive industries and are mainly based on the following four systems: Mg—Al system (i.e., AM20, AM50, AM60); Mg—Al—Zn system (i.e., AZ91D); Mg—Al—Si system (i.e., AS21, AS41); and Mg—Al—Rare Earth system (i.e., AE41, AE42).

Magnesium-based alloy cast parts can be produced by conventional casting methods which comprise die-casting, sand casting, permanent and semi-permanent mold casting, plaster-mold casting and investment casting. These materials demonstrate a number of particularly advantageous properties which have prompted increased demands for magnesium-based alloy cast parts in automotive industries. The advantageous properties include low density, high strength-to-weight ratio, good castability, easy machineability and good damping characteristics. However, AS and AE alloys, while developed for higher temperature applications, offer only a small improvement in creep resistance and/or are expensive. On the other hand, AM and AZ alloys are limited to low strength and have poor ductilities for casting.

Therefore, there is room for improvement within the art.

BRIEF DESCRIPTION OF THE DRAWINGS

Many aspects of the embodiments can be better understood with reference to the following drawings. The components in the drawings are not necessarily drawn to scale, the emphasis instead being placed upon clearly illustrating the principles of the embodiments. Moreover, in the drawings, like reference numerals designate corresponding parts throughout the several views.

FIG. 1 is a photograph demonstrating of the microstructure of a typical magnesium alloy AZ91D.

FIG. 2 is a photograph demonstrating of the microstructure of an embodiment of a magnesium alloy of the disclosure.

DETAILED DESCRIPTION

An embodiment of a magnesium alloy contains: 8.7 to 11.8 wt % aluminum (Al), 0.63 to 1.93 wt % zinc (Zn), 0.1 to 0.5 wt % manganese (Mn), 0.5 to 1.5 wt (weight) % rare earth elements (RE), and the rest being magnesium and unavoidable impurities. RE is preferably selected from the group consisting of cerium (Ce), lanthanun (La), praseodymium (Pr), neodymium (Nd), yttrium (Y), and their combinations.

Al is an element for improving the strength of the magnesium alloy. Al tends to bind with the magnesium to form significant amounts of α-phase magnesium, and β-phase Mg₁₇Al₁₂ intermetallic compound to increase the mechanical strength. The magnesium alloys of the present disclosure comprises 8.7 to 11.8 wt % Al. A magnesium alloy containing less than 8.7 wt % Al does not exhibit good fluidity properties and castability. On an another spectrum, a magnesium alloy containing more than 11.8 wt % Al tends to be brittle. A preferred range for Al in magnesium alloys of the present disclosure is between 8.8 and 10.8 wt %.

Zn is also an element for improving the strength. Zn dissolves in α-phase magnesium and β-phase Mg₁₇Al₁₂ resulting in solid solution strengthening. Zn, however, lowers the creep resistance and increases the crack sensitivity during casting. The magnesium alloys having Zn contents less than 0.63 wt % have decreased strength, castability and corrosion resistance. On the other hand, the magnesium alloys containing more than 1.93 wt % Zn is susceptible to hot tearing and are not die castable. A preferred range for the Zn content in the magnesium alloys of the present disclosure is between 0.63 and 1.02 wt %.

Mn forms an intermetallic compound with Al to improve the elongation of the magnesium alloy. To maintain a good corrosion resistance, unavoidable impurities, such as iron (Fe), copper, and nickel, in the magnesium alloys of the disclosure are kept at minimal amounts. Deteriorations of the magnesium alloys due to corrosion may be slowed by adding Mn and reducing the Fe content. A minimum amount of Mn in the magnesium alloys of 0.1 wt % is required to have significant improvement of corrosion resistant properties of the magnesium alloys. On an another spectrum, magnesium alloys having Mn more than a maximum amount of 0.5 wt %, yield ratios of melting of the magnesium alloys deteriorate.

RE tends to transform the grain boundaries of the β-phase Mg₁₇(Al, Zn)₁₂ intermetallic compounds to be smaller or discontinuous, resulting smaller grains of the β-phase Mg₁₇(Al, Zn)₁₂ intermetallic compounds. Thus, the RE is effective in improving the mechanism strength and ductility properties. A minimum amount of RE in the magnesium alloys of 0.51 wt % is required to have significant improvement of grain sizes of the magnesium alloys. On an another spectrum, magnesium alloys having RE more than a maximum amount of 1.5 wt %, castibility of the magnesium alloys deteriorate due to the RE binding with the Al to form significant amounts of Al₄RE intermetallic compound. It may result in a casting failure if the RE content exceeds the maximum amount. A preferred range for a total RE contents in the magnesium alloys of the present disclosure is between 0.51 wt % and 1.5 wt %. In addition, impurities, such as MgO, H₂, produced during making of the magnesium alloys may be reduced by adding the RE.

A method for making a magnesium alloy of the disclosure comprises following steps.

In step S1, raw materials, such as Al, Zn, Mn, RE, Mg, are melted to form a molten magnesium alloy containing: 8.7 to 11.8 wt % Al, 0.63 to 1.93 wt % Zn, 0.1 to 0.5 wt % Mn, 0.5 to 1.5 wt % RE , and the rest being magnesium and unavoidable impurities.

In step S2, the molted magnesium alloy is cast to form magnesium alloy components by casting methods, such as die-casting, thixo casting, sand casting, permanent and semi-permanent mold casting, plaster-mold casting, and investment casting and so on.

In step S3, the magnesium alloy components are re-heated to a temperature in the range of about 330 to about 420 Celsius degrees in a time in the range of about 30 to about 180 minutes. The preferred heating temperature is in the range of about 350 to about 400 Celsius degrees. The preferred heating time is in the range of 60 to 120 minutes.

In step S4, the magnesium alloy components are held for 0 to 60 minutes at the temperature in the range of about 330 to 420 Celsius degrees. The preferred holding time is in the range of 0 to 30 minutes.

In step S5, the magnesium alloy components are cooled to a room temperature.

FIG. 1 shows a photograph demonstrating of a microstructure of a typical magnesium alloy AZ91D. FIG. 2 shows a photograph demonstrating of a microstructure of an embodiment of the magnesium alloy after the heat treatment steps disclosed above. In comparison with FIG. 1, FIG. 2 shows a great amount of the β-phase Mg₁₇(Al, Zn)₁₂ intermetallic compounds in are broken and dissolved into α-phase magnesium grains. Thus, the sum of the β-phase Mg₁₇(Al, Zn)₁₂ intermetallic compounds is reduced, and a great amount of Al₄RE compounds are accumulated between the grain boundaries of the α-phase magnesium.

The magnesium alloys of the present disclosure were prepared in 100 kg crucible made of low carbon steel. The mixture of N₂+0.3% SF₆ is used as a protective atmosphere. The raw materials used were as follows:

-   -   Magnesium: pure magnesium, containing at least 99.8% Mg;     -   Aluminum: commercially pure Al (less than 0.3% impurities);     -   Zinc: commercially pure Zn (less than 0.005% impurities);     -   Manganese: in the form of Al-15% Mn master alloy;     -   Rare earth: Ce-rich rare earth.

Al is added into the molten magnesium during the melt heating in a temperature interval 650 to 680 Celsius degrees. Intensive stirring for 3-5 minutes is sufficient for dissolving this element in the molten magnesium. Zn is added into the molten magnesium during the melt heating in a temperature interval 650 to 680 Celsius degrees. Intensive stirring for 3-5 minutes is sufficient to dissolve this element in the molten magnesium. Al-15% Mn is added into the molten magnesium during the melt heating at a temperature in the range of 710 to 730 Celsius degrees. Intensive stirring for 20-30 minutes is sufficient for dissolving this element in the molten magnesium. RE is added into the molten magnesium during the melt heating in a temperature interval 690 to 710 Celsius degrees. Intensive stirring for 10-15 minutes is sufficient for dissolving this element in the molten magnesium.

After obtaining the required compositions, the molten magnesium alloys are held at a temperature in the range of 660-670 Celsius degrees, and then they were cast into the 7 kg rectangular components. The casting was performed with gas protection of the molten metal during solidification in the molds. Neither burning nor oxidation is observed on the surface of the all experimental components. The components of all new and comparative alloys were then re-melted and permanent-mold-cast into bars by JLM280MGIIe-type casting device, which are used for the preparation of specimens for tensile test.

The magnesium alloys of the disclosure have been tested by standard test method (ASTM-B557-02) and compared with a comparative sample (AZ91D). The chemical compositions of the magnesium alloys and AZ91D by ICP-AES are listed in Table 1. The results of mechanical property test of the magnesium alloys and AZ91D by are shown in Table 2.

TABLE 1 Chemical compositions of alloys (a remainder of said magnesium alloy being composed of Mg and unavoidable impurities) Alloys Al (wt %) Zn (wt %) Mn (wt %) RE (wt %) Comparative Example 8.7 0.71 0.20 — (AZ91D) Example 1 8.9 1.02 0.20 0.51 Example 2 8.9 1.02 0.20 0.51 Example 3 8.9 0.66 0.19 0.67 Example 4 9.2 0.63 0.18 0.79 Example 5 10.8 1.54 0.19 0.94 Example 6 10.8 1.54 0.19 0.94 Example 7 11.8 1.93 0.24 0.9 Example 8 11.8 1.93 0.24 0.9 Example 9 8.8 0.68 0.18 0.95 Example 10 8.8 0.68 0.18 0.95 Example 11 11.1 0.78 0.21 1.12 Example 12 9.1 0.71 0.24 1.23 Example 13 9.0 0.73 0.22 1.5

TABLE 2 Mechanical properties of alloys Whether Heat Ultimate Tensile Elongation Alloys Treatment or not? Strength (MPa) (%) Comparative Example No 240 5.3 (AZ91D) Example 1 No 251 7.8 Example 2 Yes 290 10.9 Example 3 No 253 8.6 Example 4 No 250 8.4 Example 5 No 249 5.4 Example 6 Yes 287 7.6 Example 7 No 258 4.5 Example 8 Yes 296 8.9 Example 9 No 255 8.3 Example 10 Yes 295 12 Example 11 No 245 5.3 Example 12 No 253 7.6 Example 13 No 246 5.5

As may be seen from the Tables, a great advantage of the magnesium alloys of the disclosure may be further seen when comparing them with AZ91D alloy with respect to ductility.

Finally, while the present disclosure has been described with reference to particular embodiments, the description is illustrative of the disclosure and is not to be construed as limiting the disclosure. Therefore, various modifications can be made to the embodiments by those of ordinary skill in the art without departing from the true spirit and scope of the disclosure as defined by the appended claims. 

What is claimed is:
 1. A magnesium alloy containing: 8.7 to 11.8 wt % aluminum, 0.63 to 1.93 wt % zinc, 0.1 to 0.5 wt % manganese, 0.5 to 1.5 wt % rare earth elements, and the rest being magnesium and unavoidable impurities.
 2. A magnesium alloy containing: 8.8 to 10.8 wt % aluminum, 0.63 to 1.02 wt % zinc, 0.1 to 0.5 wt % manganese, 0.51 to 1.23 wt % rare earth elements, and the rest being magnesium and unavoidable impurities. 